This invention relates to the design of secondary electrochemical cells. It is particularly applicable to cells that operate at high temperatures with molten salt electrolyte and include corrosion resistant high temperature ceramic materials such as boron nitride, yttria, silicon nitrite, aluminium nitride, calicum zirconate and magnesium oxide as cell separator materials. These materials, particularly boron nitride are quite frangible in felt, foam or compacted powder forms such that they are easily crushed, crumbled or punctured during cell operation or assembly. In previous cell designs, this material has been supported by the electrode structure often including screens or cloths disposed at electrode surfaces. The screen or cloth has the additional purpose of retaining particles of electrochemically active material within the electrodes. In order to achieve particle retention, fine mesh of e.g. 200-325 U.S. standard mesh have been selected but such screens or cloths with fine strands have been subject to ruptures followed by cell shorting due to bridging of electrode materials or stray screen wires.
Prior methods of cell assembly have involved assembling electrodes and separators into the cell housing followed by degassing under vacuum and filling the cell with molten electrolyte in a time consuming process.
Although various secondary electrochemical cells may advantageously incorporate the teachings of the present invention, those particularly contemplated include solid electrochemically active material such as alloys of alkali metals or alkaline earth metals in the negative electrode and solid transition metal chalcogenides such as iron sulfides, cobalt sulfides, nickel sulfides, etc., in the positive electrode. Cells of these types can include molten salt electrolytes such as alkali metal halides and alkaline earth metal halides or mixtures of these materials. Such cell materials are well known and are described in various patents and publications relating to this subject matter.
The following patents illustrate the general field of this development.
Arntzen, U.S. Pat. No. 4,110,517, discloses an electrochemical cell employing frangible forms of boron nitride and other ceramic materials as an electrically insulative cell separator.
Vissers et al, U.S. Pat. No. 4,029,860, describes a compartmented or honeycombed structure used as a current collector and to support electrochemically active material within the electrode of an electrochemical cell.
Kaun et al, U.S. Pat. No. 4,011,374, describes the use of a thermosetting resin as a moldable material into which electrochemically active material is blended for preparing electrodes.
Mathers et al, U.S. Pat. No. 4,086,396 discloses an electrochemical cell including a layer of powdered electrically insulative ceramic between electrodes of opposite polarity.
Eberhart et al, U.S. Pat. No. 4,189,827 teaches the use of LiAlCl.sub.4 to enhance wetting of BN by molten electrolyte salt.